Steroid hormones in association with tightly-bound receptor proteins affect the transcriptional activity of specific genes. Our long range goal is to elucidate the mechanism of transcriptional control and to identify factors that determine which genes are responsive. In this proposal wer will develop genetic and molecular approaches utilizing glococorticoid-responsive cell lines. We will select for variants with murine lymphoma cells that, like normal T lymphocytes, are killed by physiological doses of glucocorticoids. This killing response provides a powerful selection for variants. These cells also rapidly respond to glucocorticoids with increased synthesis of mouse mammary tumor virus (MTV) RNA, abiochemical response that can be used to characterize glucocorticoid-resistant cells. We will also use this response to develop independent selection schemes based on expression of MTV cell surface proteins. We have identified a cell line that is a naturally occuring variant in which MTV genes that are endogenous to the C57BL/6 mouse strain and are normally transcriptionally inactive have become hormone responsive. This variant line, as well as other variants, will be characterized in detail. We will examine genetic dominance and complementation in somatic cell hybrids and will biochemically characterize glucocorticoid receptor proteins and MTV RNA transcripts. We will also isolate variant MTV genes by molecular cloning and compare their structure to wild-type genes. Chromatin structure or gene packaging will be assessed by sensitivity to DNase I in isolated nuclei and by determining if nucleosomes are precisely placed, or "phased", along the genes. In addition, the role of DNA methylation in hormone responsiveness and the ability of cloned genes to function in DNA transformation experiments will be investigated. The proposed experiments will explore the molecular basis of developmental and regulatory phenomena potentially related to many human diseases. In addition, they will provide insight into the mechanisms of steriod resistance in lymphoid cells, a common difficulty in steriod chemotherapy. Furthermore, while a viral origin has not been implicated in any human cancer, activation of endogenous MTV genes have its counterpart in humans.